1. Field of the Invention
The present invention relates to the removal of blockages in tubular tissues and organs, and more specifically, it relates to the use of a light stimulated opto-acoustic transducer located at the end of a fiber optic within a catheter for use in ultrasound thrombolysis and angioplasty.
2. Description of Related Art
Ischemic strokes are caused by the formation or lodging of thrombus in the arterial network supplying the brain. Typically these occlusions are found in the carotid artery or even smaller vessels located still higher in the cranial cavity. Interventional cardiologists and vascular surgeons have devised minimally invasive procedures for treating these conditions in the vasculature elsewhere in the body. Among these treatments is ultrasound angioplasty whereby a microcatheter is directed to the site of an occlusion. An ultrasonic transducer is coupled to a transmission medium that passes within the catheter and transmits vibrations to a working tip at the distal end in dose proximity to the occlusion. Ultrasonic catheters for dissolving atherosclerotic plaque and for facilitating clot lysis have been described previously. Improvements on these inventions have concentrated on improving the operation or function of the same basic device (Pflueger et al., U.S. Pat. No. 5,397,301). The vibrations coupled into the tissues help to dissolve or emulsify the clot through various ultrasonic mechanisms such as cavitation bubbles and microjets which expose the clot to strong localized shear and tensile stresses. These prior art devices are usually operated in conjunction with a thrombolytic drug and/or a radiographic contrast agent to facilitate visualization.
The ultrasonic catheter devices all have a common configuration in which the source of the vibrations (the transducer) is external to the catheter. The vibrational energy is coupled into the proximal end of the catheter and transmitted down the length of the catheter through a wire that can transmit the sound waves. There are associated disadvantages with this configuration: loss of energy through bends and curves with concomitant heating of the tissues in proximity; the devices are not small enough to be used for treatment of stroke and are difficult to scale to smaller sizes; it is difficult to assess or control dosimetry because of the unknown and varying coupling efficiency between the ultrasound generator and the distal end of the catheter. Dubrul et al., U.S. Pat. No. 5,380,273, attempts to improve on the prior art devices by incorporating advanced materials into the transmission member. Placement of the ultrasonic transducer itself at the distal end of the catheter has been impractical for a number of reasons including size constraints and power requirements.
A related method for removing occlusions is laser angioplasty in which laser light is directed down an optical fiber to impinge directly on the occluding material. Laser angioplasty devices have been found to cause damage to or destruction of the surrounding tissues. In some cases uncontrolled heating has lead to vessel perforation. The use of high energy laser pulses at a low or moderate repetition rate, e.g. around 1 Hz to 100 Hz, results in non-discriminatory stress waves that significantly damage healthy tissue and/or result in insufficient target-tissue removal when the independent laser parameters are adjusted such that healthy tissue is not affected. Use of high energy laser light to avoid thermal heating has been found to cause damage through other mechanisms that puncture or otherwise adversely affect the tissue.
It is an object of the present invention to provide an opto-acoustic transducer located at the end of a fiber optic within a catheter for use in the removal of either partial or complete vascular blockage, or other luminal occlusions.
The invention has uses in therapeutic applications of angioplasty and thrombolysis. Angioplasty is the removal of atherosclerotic plaque and thrombolysis is the removal of soft clots which are typical in the brain and in the coronary arteries. The problems of energy transmission through the catheter are addressed by using an optical fiber to guide laser pulses to the distal end. Unlike laser angioplasty or laser thrombolysis, direct ablation of the occlusion is not attempted; rather, a high frequency train of small laser pulses is used as an energy source for a miniature ultrasonic transducer. This transducer can be used in a similar fashion to the prior art catheter-based ultrasonic devices. Dissolution of the occlusion is then promoted by the ultrasonic action, and not directly by the interaction with the laser light.
The use of optical energy to induce an ultrasonic excitation in the transducer offers a number of advantages. Most importantly, it can be implemented through a 2.5 French or smaller catheter, such as is required for accessing dots in the cerebrovasculature. Optical fibers can be fabricated to small dimensions, yet are highly transparent and capable of delivering substantial optical power densities from the source to the delivery site with little or no attenuation. The opto-acoustic transducer provides for variable energy delivery by means of independent control over light delivery/working fluid coupling. The working fluid is a substance contained within the transducer which absorbs the optical energy. Engineering design parameters include optical wavelength, optical energy, light pulse duration, light pulse repetition frequency, light pulse duty cycle, fiber dimensions, fiber materials, scattering and absorption of the optical energy in the working fluid, and the thermodynamic parameters of the working fluid. The additional engineering of the mechanical transducer head results in acoustic energy of appropriate magnitude and frequency such that targeted occlusions are preferentially damaged while healthy tissues remain intact or otherwise minimally affected. The present invention will allow delivery of sufficient energy to generate acoustic excitation through a small and flexible catheter, such as is required for stroke treatment.